Magnetic tunneling junction (MTJ) or Giant magneto-Resistance (GMR)/Spin Valve (SV) with two ferromagnetic layer separated by a non-magnetic layer—a tunneling oxide layer for MTJ or a transitional metal for GMR/SV—have been widely studied for use as a memory element in, for example, a Magnetic Random Access Memory (MRAM). Usually, one of these ferromagnetic layers (reference or pinned layer) is magnetized in a fixed direction while the other layer is free to switch its magnetization direction (free layer).
For MRAM applications, the magnetizations of both free and reference layers are in the film plane, as illustrated by FIG. 1a. The anisotropy field that keeps the free layer magnetization parallel or anti-parallel to the reference layer is usually generated through shape anisotropy that occurs when the shape deviates from a circle, e.g. as an ellipse. In the quiescent state, the free layer magnetization lies along the long axis of the cell (see the ellipse in FIG. 1a) oriented in the direction of magnetization of the reference layer, either parallel or anti-parallel thereto. This long axis is referred to as the easy axis (x), while the direction perpendicular to it is the hard axis (y). The cross section of FIG. 1a is given in FIG. 1b. 
The digital information stored in the MTJ is thus encoded as the direction of magnetization of the free layer. FIG. 2 shows resistance R of such a MTJ element as a function of external field Hs along the orientation of the pinned layer magnetization. When the field is off, the two states with minimum and maximum resistances correspond to the free layer magnetization being parallel and anti-parallel to the pinned layer magnetization respectively. The field required to switch between the two states (Hs) is determined by the anisotropy energy of the element.
In a conventional MRAM application, two orthogonal external fields are used to program an MRAM cell such as 35. These are provided by current lines 31 and 32, as shown in FIG. 3. The bit line provides the easy axis field while the word line provides the hard axis field. To program a cell, both bit and word line currents are applied, the combination of these two fields overcoming the shape anisotropy to set the magnetization of the selected cell into a desired direction. Although cell 35 is the one that was selected, many other cells, along either a powered bit or word line, such as 33 or 32, also experience a field from either a bit line current or from a word line current, albeit smaller than the combined field that is experienced by the selected cell. Such cells are referred to as half-select cells. They are susceptible to being accidentally programmed and thereby causing an error.
Another shortcoming of this approach is the scaling difficulty: as dimensions grow smaller, thermal agitation may perturb stored information. This thermal effect is described by
  f  =            f      0        ·          exp      ⁡              (                  -                                    B              ·                              H                s                            ·                              M                s                            ·              V                                      k              ·              T                                      )            
where f is the thermal switching frequency, f0 and B are constants, k is the Boltzman constant, T is the temperature. To have a thermally stable stored information in the MRAM cell, the Δ=BHsMstA has to be higher than a certain constant value. As dimension scales down, the area A is decreased, to maintain constant value of Δ, the
      H    s    =      Δ                  BM        s            ⁢      tA      has to be increased, hence requiring higher switching current to write.
These two shortcomings can be avoided by thermally assisting the switching of the magnetization during the write operation, as described in Ref [1] as well as in U.S. Pat. Nos. 6,385,082, 6,704,220, and 6,771,534. The latter propose using joule heating to reduce the Ms value, and hence to lower Hs, while maintaining thermal stability when this heat is absent. However, in this scheme the choice of free layer material is limited by the requirement that a large enough magneto-resistance has to be achieved for there to be enough read signal for the detection of stored information. Thus the choice of free layer is usually limited to Co, Fe, Ni, and their mutual alloys. These all have high Curie temperatures so the current required to obtain a significant reduction of Ms is very high. They suggested use of rare earth ferromagnetic materials that have low Curie temperatures and lower magneto-resistance values than Co, Fe, Ni, and their mutual alloys. But these materials and are highly corrosive which makes for great process difficulties.
The other approach to overcoming half-select and scaling issues is an exchange biased design, described in Ref. [1], for current field writing, and in U.S. Pat. No. 7,110,287 for spin torque transfer writing, to couple the free layer to a low blocking temperature (Tb) AFM layer (separate from the AFM used to pin the reference layer which has a high blocking temperature).
A schematic drawing of this design is shown in FIG. 4. Shown there are second AFM layer 41 (which has lower Tb), seed layer 42, bottom electrode 43, and diode/transistor 44. Data storage is achieved by changing the direction of the exchange-coupling field (He) of second AFM 41 at free layer 11. This is achieved by sending a current pulse through the MTJ to heat AFM layer 41 above Tb so He goes to zero. The word line is then energized to provide a directional magnetic field at the free layer, following which the heating current is turned off so that the magnetized free layer of the MTJ cools in the presence of the word line field. This sets the magnetization of the free layer in the desired direction, either parallel or antiparallel to the reference layer's magnetization.
The anisotropic field to maintain stored data against thermal agitation is provided by the unidirectional field
      H    e    =      J          2      ⁢                          ⁢              M        s            ⁢      t      where J is the exchange coupling energy per unit area which is determined by the AFM and free layer material properties.
The problem with this design is that, in order to have low Tb, the AFM's thickness has to be small; but He drops, and its variability increases, rapidly with AFM thickness so one ends up with a wide range of He values distributed among the various memory elements. The temperature generated by the heating current has to overcome the AFM with the highest Tb, which means that a high current will be needed.